1. Field of the Invention
The present invention relates to novel semi-permeable membranes, and, more especially, to semi-permeable membranes useful for ultrafiltration techniques. The invention also relates to a process for the fabrication of such membranes.
2. Description of the Prior Art
It is well known to this art to utilize semi-permeable membranes in various separation processes, such as reverse osmosis and ultrafiltration.
For such applications, it is advantageous to fabricate screen-based or reinforced membranes, namely, membranes which comprise, for at least a portion of their thickness, a flexible support or backing [which is frequently a fabric, non-woven or grid], which support renders the membranes both strong and easy to handle, even when in the form of samples having large dimensions. Screen-based membranes are generally produced by casting a polymer solution, followed by removal of the solvent [notably by evaporation and/or coagulation]. The polymer solution is hereafter referred to as the "collodion", regardless of the polymer envisaged. If the collodions used are too dilute, it is difficult to obtain a uniform coating; if the thickness of the cast collodion is large, the collodion tends to spread and overflow, and this leads to a nonuniform thickness, whereas, if the thickness of the cast collodion is small, there is a risk of not covering the screen completely. Furthermore, in terms of the coagulation following the casting of the collodion, the inevitable ripples which occur on the surface of the coagulating bath form surface irregularities [wrinkling] on the membrane; too, the polymer [as a dilute collodion] coagulates incompletely and elements become detached therefrom and denature the coagulating liquid by forming suspensions thereof.
It is therefore of the greatest value, industrially, to be able to make use of fairly concentrated polymer solutions.
For the production of semi-permeable membranes, a very large number of known polymers have been tested or utilized. Polymers based on polysulfone derivatives are of value essentially because of their properties of enhanced chemical stability which are inherent by reason of the very nature of polysulfones, and independently of the fact that same have been shaped as a membrane.
Thus, French Certificate of Addition No. 2,005,334 has described polysulfone membranes which are themselves known, as per French Pat. No. 1,584,659. These membranes are fabricated from a collodion containing from 8 to 20% of polymer. If it is desired to produce polysulfone membranes from more concentrated solutions [for example, of at least 25% strength, so as to give a viscosity greater than at least 15 Poises], it is no longer possible to obtain membranes which can be used in ultrafiltration.